Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19.
The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts 26, 28, 30.
In order to improve design as well as confirm performance it is advantageous to inspect parts of a gas turbine engine during operation. It is known to use such techniques as boroscoping which acts in a similar manner to an endoscope and so allows images to be obtained. Generally such inspection tools include a flexible or rigid shaft which is inserted into an engine or through a suitable inspection port. Observations are then made remotely either directly by looking down a scope into an inspection volume or indirectly by attaching a camera onto the exposed end of the endoscope and observing the image on a TV camera. It will also be understood alternatively a camera can be placed at the distal end of the scope and steered into a viewable position by appropriate manipulation of the associated flexible or rigid shaft in such circumstances acting as a positioning tool. It will also be understood that such areas may be dark or inappropriately lighted and in such circumstances a lighting element will also be provided at the distal or steerable end of the scope.
Unfortunately use of scopes has limitations in terms of their action. It will be understood that steering the distal end through which images are viewed either directly or indirectly requires manipulation of the scope. It is difficult to steer a scope with an overall length of action which is greater than approximately five meters. These difficulties are due to:
a) Steering of the scope is via a near point hand piece which will act conveniently over a large distance, such as two to five meters. The weight of the end manipulated becomes effectively greater and so handling becomes progressively more difficult;
b) There is a problem of fouling, trapping, catching or dragging of the scope sheath as it progresses to a desired observation point;
c) It will be understood that the sheath of the scope may fall into voids and in such circumstances the scope may become stuck;
d) Obstacles in the path to a desired observation point may require steering with relatively severe curvatures in the scope sheath which again can cause seizing of the probe in use;
e) Generally there are a limited number of access points into an engine and in such circumstances it is quite common to need to have an abrupt change in direction of the scope in order to gain access to the desired observation point again creating severe difficulties with regard to jamming of the probe. One component which has been found to be particularly difficult to access using conventional arrangements is the 6th stage of the high pressure compressor 14 (HP6). This is particularly problematic, as the HP6 may need to be regularly inspected.
In view of the above it is generally difficult to place a scope with its distal end at a desired observation area. Furthermore even with highly skilled operators it can be difficult to avoid problems particularly as situations may occur that are highly stressful in terms of personal performance such as with regard to an onsite inspection of a failing engine. As indicated a particular problem is seizure of the scope within the engine which may require costly and time consuming dis-assembly of the engine to remove the inspection tool. These problems can be further exacerbated as the engine may be observed in operation and therefore will go through thermal and mechanical cycling leading to tolerance clearance changes along the path of the scope and so possible problems with respect to seizure being exacerbated.
One known solution is to attach an inspection tool to a rotatable component of the gas turbine engine using a positioning tool, releasing the camera from the positioning tool, rotating the rotatable component and wirelessly relaying images to the operator. Such a solution is described in applicant's previous patent application, European patent specification EP2119875 (the contents of which are hereby included by reference).
However, once released, the position and orientation of the inspection tool must be maintained in order to ensure that the correct observation area is inspected, and that the inspection tool can be retrieved following the inspection. In some cases, the inspection tool may have to deployed in different areas, having different internal dimensions. It is also desirable that the inspection tool can be deployed and recovered without requiring further control wires and/or power to be supplied through the inspection port.
The present invention describes an inspection arrangement and a method of inspecting a gas turbine engine which seeks to overcome some or all of the above problems.